Shown in FIG. 2 is the general configuration of a known type of EDM die sinking machine. A wire cutting machine differs from a die sinking machine actually only in details, but nevertheless most manufacturers make use of totally different concepts for the two types of machines. This applies particularly to the pulse generator involved, where very short but high discharge pulses are needed for wire cutting, whilst for die sinking longer discharge pulses of corresponding lower amplitude are used. To date there is still no satisfactory solution for a consistent overall concept.
The configuration of an EDM system generally involves the sub-systems: AC mains input 1, power cabinet 2, cable system 3 and machine 4. The power cabinet 2 houses an AC voltage module (AC), a DC voltage module (DC), a numerical control (CNC), one or more drive modules (Drive), a generator module (Gen.) as well as a universal machine control module (Control). Since the full content of the power cabinet 2 is considerably bulky and weighty and the total power loss is of the order of a single-digit kW, the power cabinet is normally sited some distance away from the machine 4.
The cable system 3 is usually 2 m to 5 m long. A first cable connects the drive modules (Drive) to the axis drive motors of the machine 4 and supplies the motor current, the current for any brakes as may be provided as well as diverse sensitive digital signals of the position transducers. These cables are a significant cost factor and if not designed with due care can easily result in expensive downtime.
A second cable connects the generator module (Gen.) to the work piece and to the electrode of the machine 4. This second cable has the disadvantage that the power losses, particularly in wire cutting, due to the high RMS value of the pulse current may be as high as 100 W/m. Apart from this undesirable waste of energy this can also result in the machine structure becoming twisted from the heat and thus to work piece inaccuracies. Currently, the only solution to this problem is a complicated means of water cooling.
Another disadvantage is also involved in the high rigidity of the cables used, typically needing to involve eight coaxial cables in parallel, each of approximately 5 mm2 copper section. Since the cables are connected to moving structure parts of the machine, their rigidity results in flexing of these structure parts in the micrometer range and thus, of course, to corresponding errors in work piece machining.
A third cable serves to connect the universal machine control module (Control) to a large number of function units on the machine 4, such as electrovalves, pumps, auxiliary drives, end switches, temperature sensors, safety guards, etc. This third cable likewise results in considerably costs because a great many different conductors are needed, but also because each machine variant needs ultimately a special cable. A further disadvantage materializes when the machine 4 and the power cabinet 2 are shipped separately to the customer, the many connections of the cable system 3 required on installation constituting an added fault risk.
In the Proceedings of the 13th ISEM Vol. 1, Bilbao 2001, pages 3 to 19 all processes and equations fundamental to pulse generation via pulse capacitors are explained as regards their application in micro EDM. These comments apply in general and thus also to the present invention.
In the Proceedings of the 13th ISEM Vol. 1, Bilbao 2001, pages 153 to 160 a dual half-bridge type non-resistive generator is explained. This generator is designed so that each half-bridge can generate symmetrical delta currents. When suitably controlled, the sum of the currents of the two half-bridges is a zero-ripple trapezoidal pulse. By pulse width modulation with a signal representing the current shape within the range of the rise and fall times of the delta currents a great variety of desired current shape can be synthesized. Since only half-bridges are provided, the pulse shape at the output can correspondingly only be monopolar. Although eliminating the load resistors improves efficiency, this is almost instantly reduced because of the commutations during the current peaks. This disadvantage becomes all the more serious the higher the pulse current and the frequency are selected. When such a generator is used for generating steep pulses, as is usual, high frequencies are necessarily required. A further problem lies in a disadvantage of the bridge circuit itself, namely in the existence of switching elements between the work piece and the power supply, it being between these points that the steep commutation flanks result in high displacement currents at the AC mains side, ultimately resulting in poor electromagnetic compatibility. For the same reason, the two sources need to be DC decoupled from each other which unnecessarily adds to the costs of the configuration.
U.S. Pat. No. 4,710,603 discloses a generator, operating on the pulse capacitor discharge principle, the basic circuit of which is shown in FIG. 3. From a DC voltage source E a capacitor C1 is charged via a switching element Q1 and an inductance L3. A further switching element Q2 discharges the pulse capacitor C1 via a further inductance L2 into the spark gap PW. This circuit requires neither charge resistors nor switching elements in linear operation.
U.S. Pat. No. 4,766,281 discloses a generator with a passive charging voltage regulator as shown in FIG. 4. The charging voltage regulator comprises a flyback converter transformer and two diodes. The efficiency of this generator is high since the commutation losses as occur with the generator as it reads from U.S. Pat. No. 4,710,603 across the switching element Q1 are eliminated.
However, both generators still have disadvantages. Firstly, the pulse frequency is restricted to modest values of around 70 kHz due to monopolar charging. Increasing the frequency further would allow the charge current to increase to values adversely affecting the efficiency. Secondly, the generators are still too large to permit their location e.g. in the direct vicinity of the electrode. For a more detailed explanation of this, reference is made to FIG. 5 plotting for these generators the curves of the capacitor voltage Uc and pulse current Igap at the spark gap as a function of time t. It is evident that for a sinusoidal pulse current Igap the negative charging voltage U_chrg flips cosinusoidally to a positive residual charging voltage U_end. This residual charging voltage U_end corresponds precisely to the energy which is not converted in the spark gap and reflected back to the pulse capacitor. Ignoring the line losses the residual charging voltage as it reads from the aforementioned Proceedings of the 13th ISEM Vol. 1, Bilbao 2001, pages 3 to 19 is:U_end=−U_chrg+2*U_gap  (1)where U_gap corresponds to the voltage across the spark gap. The residual voltage U_end is accordingly a function of neither the pulse current nor of the capacitance of the pulse capacitor, nor of the inductance of the discharge circuit. After a discharge the charging voltage regulator immediately commences to recharge the pulse capacitor again to the desired negative charging voltage U_chrg. In this arrangement, the complete electrical energy of the residual charging voltage U_end is converted within an inductance (e.g. within the coil L3 in FIG. 3 or within the flyback converter transformer in FIG. 4) firstly into magnetic energy, before then being stored again in form of electrical energy in the pulse capacitor in reverse polarity.
U.S. Pat. No. 6,525,287 B2 discloses a further generator including a plurality of capacitors for pulse generating. The capacitors are inserted in the cross-branch of a full bridge in AC operation. The main idea of this invention is replacing the load resistors by the lossless AC impedance of the capacitors. Here, however, the switching elements of the full bridge need to commutate practically the total load current with high losses otherwise the ripple on the current would be 100% and a considerable amount of energy is inductively stored in the cable system. To safely handle such amounts of energy additional lossy safety circuits are needed.
U.S. Pat. No. 5,380,975 discloses a generator including a plurality of pulse capacitors which are simultaneously discharged by switching elements into the spark gap. This results in an increase in the capacitance as compared to that of a single pulse capacitor and the discharge energy can be maintained at a prescribed level.
U.S. Pat. Nos. 4,072,842 and 6,281,463 show generators wherein ignition voltage sources are connected via diodes to a switching mode type generator for making available a selective high ignition voltage at the spark gap whilst retaining a minimized switching frequency of the pulse current regulator. All ignition sources of these generators comprise load resistors and are unable, for example, to regulate the pulse current or otherwise influence its shape. Although generators of this kind were an advancement as long as the ignition voltage source was only used to generate a voltage (i.e. essentially without current) when attempting to also use the ignition voltage source for generating the smaller discharge currents (problematic for DC regulators because of the switching frequency being too high), however, the principle produced an unwanted drop in efficiency.
The object of the invention is to provide a method and a generator for efficiently generating discharge pulses having a freely selectable ignition voltage.